In a conventional telephone and radio multiplex system there may be transmitted simultaneously a plurality of narrow band carrier signals, each having a predetermined center frequency, e.g., 88 kHz.+-.10 Hz. In a typical frequency bandwidth of 88-160 kHz, the plurality of carrier signals in such a system may be transmitted with a relatively close spacing, e.g., 4 kHz. At some point in such a system, there is a need to select from among the described plurality of narrow band closely spaced carrier signals, a carrier signal having a particular center frequency. For example, a pilot signal, which is sent in the same band as a group of modulated carrier signals, is detected and used by transmission equipment for a variety of functions.
Heretofore, the selection of a carrier signal from among a plurality of other signals having closely-spaced center frequencies has required the use of costly and complex circuits. Typical of such circuits is a circuit comprising a conventional high Q, LC bandpass filter network and operational amplifier assembly.
With the ever-increasing demand on the providers of communication systems to reduce costs, the need for circuits of small size and low power consumption, and the expanding use of solid state circuits in communication networks, the elimination of such costly and complex conventional circuit components as the aforementioned LC-operational amplifier assembly, is desirable.